What is the difference between autopilot and fly-by-wire system?

Question

I observed a lot of people misinterpret these words. In my opinion, autopilot assists the pilot by inputting the commands to the computer whereas fly-by-wire (FBW) system sends and receives signals from control surfaces.

• I have heard Airbus's FBW system overrides the pilot commands most of the time. Does this imply that the aircraft is controlled by a FBW system? What about Autopilot?
• When aircraft encounters sudden gusts, does autopilot contribute to making the aircraft stable? I know FBW will send signals to the different control surfaces to act accordingly.
• Which system controls the aircraft and offers carefree maneuvering? Which system assures the aircraft will remain safe and stable at every point?

In bullet 3, I believe both systems make sure the aircraft remain safe and stable. The autopilot makes landing, takeoff, and flying safe and easy with the help of ILS, autothrottle, etc. and FBW assures that every control surface is actuated in a safe manner which means it is related to attitude control, right? How do both systems work hand in hand?

Answer 1

An autopilot can fly the aircraft based on pilot selected modes. The autopilot therefore reduces the pilot's workload by replacing manual inputs via the yoke or sidestick. A fly-by-wire (FBW) system is a type of control system where mechanical connections between the flight controls and the control surfaces are replaced by electrical connections. Usually, there is also a computer involved in between which can adjust or restrict the signals sent to the control surfaces.

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To answer your specific questions:

I have heard Airbus's FBW system overrides the pilot commands most of the time.

The Airbus FBW system is active all of the time, but it does not override the pilot input most of the time. It will only override pilot input, if such input would get the aircraft out of its nominal flight envelope. The FBW system therefore provides flight envelope protections. These protections are however not active in every control law (see busdriver's answer).

Does this imply that the aircraft is controlled by a FBW system? What about Autopilot?

The aircraft is controlled by the pilot or autopilot via the FBW system. The pilot or autopilot (A/P computer) send their input electrically to the flight control (F/CTL) computers, which then actuate the control surfaces based on the current control law:

^{(Airbus A320 FCOM - Flight Controls - Description)}

Since both pilot and autopilot inputs are going through the flight control computers, all flight envelope protections are still provided by these computers, even if the autopilot is active.

When aircraft encounters sudden gusts, does autopilot contribute to making the aircraft stable?

All transport category aircraft must be statically and dynamically stable, even if the autopilot is turned off. The FBW system may however assist in achieving the stability (e.g. in yaw damping, see this answer).

If you mean maintaining a constant heading and altitude in gusty conditions, that must be done by the pilot or autopilot.

Which system controls the aircraft and offers carefree maneuvering?

Control is either with the pilot or the autopilot (never both: if the sidestick or rudder pedals are moved beyond some threshold, the autopilot will disconnect). I'm not quite sure what you mean with "carefree maneuvering", but all flight envelope protections and other assists (like e.g. automatically maintaining altitude when banking, see this answer) are provided by the FBW computers.

Which system assures the aircraft will remain safe and stable at every point?

As long as the flight envelope protections are active (depending on control law), the FBW will make sure the aircraft remains safe. In degraded control laws (especially in direct law), the pilot is still responsible for keeping the aircraft safe and stable.

The autopilot does not really care for safety though: if you tell the autopilot to climb at 6000 ft/min at cruise altitude, it will happily attempt to do so, but the flight envelope protections will kick in before the airspeed drops too much and the aircraft stalls.

Answer 2

Fly-by-wire system removes physical link between cockpit controls and flight control surfaces. To do this, there must be some sort of rule how pilot's input is interpreted and converted into control surface movement. This is called a control law.

In Airbuses there are three* different kind of control laws. "Normal law", "alternate law" and "direct law". In normal law side stick inputs are interpreted as G-load demand and roll rate. In direct law there is direct relation between side stick position and control surface deflection. In normal law there is also flight envelope protection, in direct law there is no protections what-so-ever. Still they all are fly-by-wire. I've flown several different airbuses for many years, and not once I've had a malfunction which would have degraded control law from "Normal law" nor not once has fly-by-wire system restricted my manual control input.

The flight envelope protection is there do exactly what the name implies: it will not let you stall or overload the structure. If you stay within the designed envelope you will get the full performance the aircraft has to offer. Even more, if you need the maximum available performance for any reason, like to suddenly avoid a mountain top, you can pull side stick back and let the computer do its magic without fear of over stressing the airframe.

The autopilot is there to control the pre-programmed flight path, ie. maintaining altitude, turning and maintaining to specified heading or tracking the flight plan.

The difference is that for example if sudden gust causes aircraft to climb for example 100ft, the autopilot will return to the preassigned altitude. If you are flying the same flight path in manual control the same gust will leave you in the new altitude and a pilot input is required to return to the desired flight path.

If autopilot fails, the flight can be operated with manual control with all the goodies available. Then again, if critical control computers should fail it might be that besides degraded control law the autopilot is not working either. The autopilot drives the control surfaces through the same computers as the pilot.

*There are more, and mixtures of different modes but this is the basic concept

Answer 3

You have already received a number of excellent answers, and I just want to add something very short and to the point, regarding the question you pose in your title:

What is the difference between autopilot and fly-by-wire system?

Autopilot refers to who is in charge of the flight control inputs, Fly-by-wire refers to how flight control inputs translate into control outputs. The autopilot makes its own inputs, FBW translates inputs it gets from someone else (either the pilot or the autopilot) into outputs.

Without FBW, there is typically some form of direct mechanical linkage between the flight control inputs (yoke, pedals, levers) and control outputs (aerodynamic control surfaces, throttles, etc.) With FBW, there is some form of electronic or computer support involved. The extreme case is that there is no direct link at all: a sensor in the yoke measures the angle and sends a command to a motor on the elevator, for example.

FBW does not make control inputs on its own, it only translates control inputs into control outputs.

The autopilot makes its own control inputs. This does not require any FBW at all. There are autopilots which simply pull on the same cables that the pilot does. Even though this is not typically how it works, you can think of it as follows: no matter how simple and mechanical your airplane is, in the worst case, the autopilot could always simply consist of a set of motors that do exactly what you would do – you could even have an autopilot for a hang glider that shifts some weights around on ropes. Or, you know, this.

Answer 4

fIn short

A very simple way to understand the difference between autopilot and fly-by-wire is to compare them to an autonomous car and its anti-skid system.

A more aeronautical way is to look at how a turbine engine is actually controlled: The pilot gives their orders using a throttle, but orders are overridden by the FADEC on the engine, which actually manages the engine controls. Even when the auto-throttle is on and orders come from a computer instead of a pilot, the FADEC still intercepts them. In both cases:

• The autonomous car computer and the auto-throttle are autopilot functions.
• The anti-skid and FADEC are FBW functions.

They live their lives independently of each other, exactly like the autopilot and the control surfaces FBW computers live independently of each other.

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Autopilot is a pilot

Autopilot, or flight computer, is comparable to a substitute for the pilot manual inputs. What does the flight computer could be done by manual inputs. However why would you send manual orders to fly a holding pattern if a computer can do it automatically, managing wind, speed, bank angle limits, etc, itself?

For Airbus the autopilot is the Flight Management Guidance System (FMGS).

Flight-by-wire is a elaborate anti-skid system

FBW is in my opinion a very bad choice for the concept it covers, as it introduces the idea there are wires and electronics between the pilot hand or foot and the control surface actuator.

There are actually wires and electronics, but wires are not a requirement, this could be a mechanical linkage assisted by electronics. And actually in Airbus FBW there are both wires and mechanical linkages.

The point of a FBW system is the orders received, regardless of whether they come from the pilot or the flight computer, are "managed" by a flight control computer. In Airbus avionics, the crew and the FMGS (autopilot) send commands to redundant computers:

• 2x ELAC (elevator aileron computer) for elevators/THS and ailerons,
• 3x SEC (spoilers elevator computer) for elevators/THS and spoilers,
• 2x FAC (flight augmentation computer) for rudder,

^{Airbus fly-by-wire, Source}

Computers send commands to actuators. ELAC, SEC and FAC have nothing to do with the FMGS, the computer used for autopilot functions. FGMS inputs are visible above at ELAC and FAC level.

To simplify: FBW is like an anti-skid system. The brakes on a car may receive a command which is too hard, and known to lead in certain conditions to skidding and a less efficient braking force than a smoother order. The brake computer (anti-skid) will then overrides driver inputs and translates the original order into a more efficient brake command.

The computer is able to react very quickly and to analyze actual braking in order to modulate the command. The role of this anti-skid system is independent of how the order was sent, by a human or a computer.

FBW computers are the anti-skid systems used to control rudder, elevators, ailerons and other control surfaces. They maintain the aircraft in its flight envelope, that is in the domain it is able to fly safely and efficiently, and where the control surfaces are operative (e.g. if the airspeed is too slow on the ailerons, they will be stalled, the flight envelope protection acts to prevent such possibility).

FBW laws

The way FBW work is based on ELAC and FAC being operational and receiving inputs from a lot of sensors, like the ADIRU (air data and inertial reference). If one of these computers fails or lacks inputs, then their roles is redistributed in a degraded way, and if nothing works, then control is returned to the pilot. This progressive "de-automation" is done by switching the "flight laws". Degraded laws are:

• Alternate law (two levels: With and without reduced protections),
• Direct law (no protection),
• Mechanical (mechanical linkage between controls and control surfaces).

^{Airbus flight control laws, source}

See Is a Control Law Degradation in Airbus Planes displayed on the ECAM? for more.

Answers to your questions

1. Yes most of the time autopilot (Airbus FMGS) is active, and therefore send orders to control surfaces to fly according a pre-determined route. It manages track errors.

2. Yes the flight control computers (Airbus ELAC, FAC, SEC) always override pilot inputs. This is still true when the FMGS actually pilots the aircraft. (This is comparable to car drivers orders always overridden by the anti-skid, even when a computer drives an autonomous car.)

3. In case the flight control computers fail partially or totally, some degraded ways to override pilot inputs are used, in order to continue protecting the flight envelope as most as feasible. In last resort some functions are completely returned to the pilots: This is the mechanical linkage step. In that case a pilot is able to manually control rudder and elevators.

Note one axis (pitch, roll, yaw) can be under some law and another under another law. So it is incorrect to say an aircraft is under alternate law in most cases.

Answer 5

The question seems based at least partly on the premise that "autopilot" and "fly by wire" are magical electronic computerized systems. While that is true with modern Airbus (and I believe some but not all recent Boeing models), that is not always the case, particularly with smaller airplanes.

Autopilot, in some fashion, has been around for over 100 years. While modern autopilots can take a series of waypoints and other directions and "do it all", even landing the plane, more basic autopilots manage some key metrics (e.g., speed, altitude, bank angle) and adjust engines and control surfaces to keep the plane on track. They can do the basic functions without any digital computers at all using mechanical and hydraulic linkages between controls set by the pilot and control surfaces and engines.

Fly-by-wire really has two main components, in my limited understanding. It uses electronic connections over wires (no, you don't control your elevators via WiFi from a tablet in the cockpit, I hope) in place of mechanical and hydraulic linkages, providing weight savings (which means less fuel and/or more cargo) and some other physical advantages. This doesn't technically need any computers - a simple dimmer switch could (in theory) be set up to control some remote part of an airplane over a couple of wires with no computers (not even a microcontroller) involved.

It also includes (in all current implementations that I know of, but I'm not an expert) a computer in the middle that interprets the inputs to drive the outputs, allowing for envelope protection and other features to be included (as very well explained in other answers). You could have fly-by-wire that is simply a translation of mechanical/hydraulic linkages into electrical connections. But once you are already doing that (which requires new designs and certification), it makes sense to include the computer and all the safety and efficiency features it can provide. That doesn't make as much sense on a small aircraft due to the relatively fixed cost of adding the computer (actually multiple computers for redundancy), but in a small aircraft the mechanical and hydraulic linkages are not as big a deal.

An automobile comparison is cruise control. Cruise control has been around for over 100 years on cars, and even longer on trains, long before there were any digital computers. On the other hand, while most cars are still not fly-by-wire (they have mechanical/hydraulic linkages between the steering wheel and pedals and the engine, wheels and brakes - but with power assist, much as airplanes have had for a long time), cruise control is usually computerized now to at least some degree.